Fourier transformation using an electroabsorptive CCD spatial light modulator

The use of a GaAs CCD as a spatial light modulator is described and its application to coherent optical Fourier transformation is analyzed. In this device, the transmission through the two-dimensional buried-channel CCD may be electroabsorption modulated near the GaAs cutoff wavelength since the electric field in each storage well is controlled by the transferred charge. One of the primary advantages of this modulator is the ability to electrically address the device at high speed. Analysis of the two-dimensional modulator with a silicon CCD detector array yields a projected dynamic range approximately equal to the number of modulator array elements. For arrays containing greater than 1000 elements, detector performance and nonuniformities can limit the maximum range to 30-40 dB. The device can also be optically addressed, and in this mode of operation it has a comparable dynamic range to the electrically addressed structure with an optical write energy an order of magnitude lower than liquid crystal or photorefractive light valves. An alternative mode of device operation is a waveguide mode in which the light propagates along an epitaxial layer and is modulated as it passes under a one-dimensional CCD. The detection is done by a second linear CCD. The higher modulation efficiency results in a dynamic range approximately one hundred times the number of elements but is again limited to 30-40 dB because of detector response and nonuniformities.